A unit (33) for generating count images for separate energy windows generates main measured count images and auxiliary measured count images on the basis of gamma ray (6) count information measured by a detector head (10). A main measurement window direct ray count rate estimation unit (42) estimates a count rate for direct gamma rays in a main measurement energy window, doing so by subtracting a scattered gamma ray count rate for an auxiliary measurement energy window, which has been estimated from an auxiliary measured count image and detector response data by an auxiliary measurement window scattered ray count rate estimation unit (41), from the main measured count image.